A synchronizing mechanism (1) having insert-shifting keys (4,4) comprises an input shaft (2) which is a spline shaft, a synchro-hub (3) which is put on said input shaft (2) and has a plural number of key attachment notches (31,31) on the outer surface, insert-shifting keys (4,4) which are fitted into said key attachment notches (31,31) of said synchro-hub (3), a coupling sleeve (5) which is slidably put on said synchro-hub (3) and has three key fitting recesses (51,51) formed on said coupling sleeve's (5) inner surfaces to fit said insert-shifting keys (4,4) therein, synchro-springs (6,6) which act to guide said insert-shifting keys (4,4) toward said coupling sleeve (5), a synchronizer ring (7) which is put on said input shaft (2) adjacent to said synchro-hub (3) and has a boss part (70) which has a cone shaped inner surface (71), and a synchronized gear (8) which is put on said input shaft (2) adjacent to said synchronizer ring (7) and has a boss part (80) which has a cone shaped friction surface (81) on the outer surface contacting with the inner surface (71) of said synchronizer ring (7).
Hitherto, for example, C-shaped synchro-springs (6A,6A) which are shown in FIG. 7 and FIG. 8 have been provided as said synchro-springs (6,6). Said C-shaped synchro-springs (6A,6A) are fitted into grooves (32,32) which are formed on the both sides of said synchro-hub (3), and are deformed elastically to contact with spring holding recesses (41,41) which are formed on the inner part of said insert-shifting keys (4,4), so that said C-shaped synchro-springs (6A,6A) act to guide said insert-shifting keys (4,4) toward said coupling sleeve (5).
However, in the prior art described above, d1 should be larger than d2 (d1>d2) to contact said synchro-springs (6,6) with spring holding recesses (41,41) of said insert-shifting keys (4,4) certainly, wherein d1 is a radius of grooves (32,32) of the synchro-hub (3), d2 is a distance from the inner surface of spring holding recesses (41,41) of the insert-shifting keys (4,4) to the center of the shaft before sliding of said coupling sleeve (5), d3 (<d2) is a distance from the inner surface of said insert-shifting keys (4,4) to the center of the shaft after sliding of said coupling sleeve (5), d4 is a radius of the outer surface of the key groove (72) of the synchronizer ring (7), and d5 is the effective radius (average radius) of the inner surface (71) of said synchronizer ring (7). Further, d3 should be larger than d4 (d3>d4) to keep a space between the inner surface of said spring holding recesses (41,41) of said insert-shifting keys (4,4) and the outer surface of said key grooves (72,72) of said synchronizer ring (7). And further, d4 should be larger than d5 (d4>d5) to ensure the strength of said synchronizer ring (7). Accordingly, a large d5 cannot be settled, so that it is difficult to ensure a enough capacity of the synchronizer ring.